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  • Why is there no parameter contra-variance for overriding?

    - by Oak
    C++ and Java support return-type covariance when overriding methods. Neither, however, support contra-variance in parameter types - instead, it translates to overloading (Java) or hiding (C++). Why is that? It seems to me that there is no harm in allowing that. I can find one reason for it in Java - since it has the "choose-the-most-specific-version" mechanism for overloading anyway - but can't think of any reason for C++. Example (Java): class A { public void f(String s) {...} } class B extends A { public void f(Object o) {...} // Why doesn't this override A.f? }

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  • Transferring object from parent to child

    - by waiwai933
    I'm currently developing an Custom Application using the IP.Board framework, which is in PHP, which by default, creates a IPSMember object for the logged-in user. However, I'm developing an additional class, basically class SpecialUser extends IPSMember Is there a way to get the parent object, which is IPSMember to change to SpecialUser?

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  • MustOverride Shared Methods in .Net

    - by Ben
    Hi I have a few class's that inherit from a base class. I want to enforce that all of these Class's implement a shared method foo(). I am not able to create the following on the base class Public MustOvveride Shared Sub foo() as this is not allowed. I also cant create an Interface that defines a Shared Sub Foo() and implement that in my derrived class's, as this also is not allowed. Does anyone know how i should implement this? p.s i realise my example is in VB and i have also tagged C#, the same problem applies to both languages and presumably the same fix would apply.

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  • 2 Classes need each other declared C++

    - by Prodigga
    I have a "Game" class which holds all the games settings and manages the game. I have a "Grid" class which is the grid the game is played on. The "Game" class initializes a "Grid" object as one of its members (passing itself ("this") as one of the parameters for "Grid"s constructor).. The "Grid" object therefor needs to deal with a "Game*" pointer. To do this it needs to know what "Game" is; i need to declare it before "Grid". But "Game" uses "Grid"...so it also needs "Grid" declared before it. so confused on how to include headers/etc correctly here..

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  • Is there anything bad in declaring static inner class inside interface in java?

    - by Roman
    I have an interface ProductService with method findByCriteria. This method had a long list of nullable parameters, like productName, maxCost, minCost, producer and so on. I refactored this method by introducing Parameter Object. I created class SearchCriteria and now method signature looks like this: findByCriteria (SearchCriteria criteria) I thought that instances of SearchCriteria are only created by method callers and are only used inside findByCriteria method, i.e.: void processRequest() { SearchCriteria criteria = new SearchCriteria () .withMaxCost (maxCost) ....... .withProducer (producer); List<Product> products = productService.findByCriteria (criteria); .... } and List<Product> findByCriteria(SearchCriteria criteria) { return doSmthAndReturnResult(criteria.getMaxCost(), criteria.getProducer()); } So I did not want to create separate public class for SearchCriteria and put it inside ProductServiceInterface: public interface ProductService { List<Product> findByCriteria (SearchCriteria criteria); static class SearchCriteria { ... } } Is there anything bad in this interface? Where whould you place SearchCriteria class?

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  • Perl - Calling subclass constructor from superclass (OO)

    - by Emmel
    This may turn out to be an embarrassingly stupid question, but better than potentially creating embarrassingly stupid code. :-) This is an OO design question, really. Let's say I have an object class 'Foos' that represents a set of dynamic configuration elements, which are obtained by querying a command on disk, 'mycrazyfoos -getconfig'. Let's say that there are two categories of behavior that I want 'Foos' objects to have: Existing ones: one is, query ones that exist in the command output I just mentioned (/usr/bin/mycrazyfoos -getconfig`. Make modifications to existing ones via shelling out commands. Create new ones that don't exist; new 'crazyfoos', using a complex set of /usr/bin/mycrazyfoos commands and parameters. Here I'm not really just querying, but actually running a bunch of system() commands. Affecting changes. Here's my class structure: Foos.pm package Foos, which has a new($hashref-{name = 'myfooname',) constructor that takes a 'crazyfoo NAME' and then queries the existence of that NAME to see if it already exists (by shelling out and running the mycrazyfoos command above). If that crazyfoo already exists, return a Foos::Existing object. Any changes to this object requires shelling out, running commands and getting confirmation that everything ran okay. If this is the way to go, then the new() constructor needs to have a test to see which subclass constructor to use (if that even makes sense in this context). Here are the subclasses: Foos/Existing.pm As mentioned above, this is for when a Foos object already exists. Foos/Pending.pm This is an object that will be created if, in the above, the 'crazyfoo NAME' doesn't actually exist. In this case, the new() constructor above will be checked for additional parameters, and it will go ahead and, when called using -create() shell out using system() and create a new object... possibly returning an 'Existing' one... OR As I type this out, I am realizing it is perhaps it's better to have a single: (an alternative arrangement) Foos class, that has a -new() that takes just a name -create() that takes additional creation parameters -delete(), -change() and other params that affect ones that exist; that will have to just be checked dynamically. So here we are, two main directions to go with this. I'm curious which would be the more intelligent way to go.

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  • Singletons and other design issues

    - by Ahmed Saleh
    I have worked using different languages like C++/Java and currently AS3. Most applications were computer vision, and small 2D computer games. Most companies that I have worked for, they use Singletons in a language like AS3, to retrieve elements or classes in an easy way. Their problem is basically they needs some variables or to call other functions from other classes. In a language like AS3, there is no private constructor, and they write a hacky code to prevent new instances. In Java and C++ I also faced the situation that I need to use other classe's members or to call their functions in different classes. The question is, is there a better or another design, to let other classes interact with each others without using singletons? I feel that composition is the answer, but I need more detailed solutions or design suggestions.

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  • How can I handle multiple views of a data object? Which design pattern is acceptable?

    - by tranquil.byte
    I have a person object. class Person { private $name; ... } I need to be able to change how they are displayed on the front-end ( visitors have control ). They can choose list view, grid view, photo view for example. class PersonDisplay { public function displayList() { // Query database // Output html to display in list mode } public function displayPhoto() { // Query database // Output html to display in photo mode } } Is this an acceptable way to handle the presentation of the items on the front-end or is there a specific design pattern I should be researching to help me with this task? Does anyone have any suggestions or ideas where this could go wrong or if this could potentially make maintenance a nightmare? The Person object was just an example very similiar to what I am using.

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  • Actionscript 2.0 Functions problem

    - by Joshua
    I have two problems. The first problem is with the following functions; when I call the function in (enterFrame), it doesn't work: onClipEvent (load) { function failwhale(levelNum) { _root.gotoAndStop("fail"); failFrom = levelNum; } function guardSightCollision(guardName, guardSightName) { if (_root.guardName.guardSightName.hitTest(_x, _y+radius, true)) { failwhale(1); } if (_root.guardName.guardSightName.hitTest(_x, _y-radius, true)) { failwhale(1); } if (_root.guardName.guardSightName.hitTest(_x-radius, _y, true)) { failwhale(1); } if (_root.guardName.guardSightName.hitTest(_x+radius, _y, true)) { failwhale(1); } } } onClipEvent (enterFrame) { guardSightCollision(guard1, guard1Sight); } Why doesn't it work?... The second problem lies in the failFrom variable: function failwhale(levelNum) { _root.gotoAndStop("fail"); failFrom = levelNum; } How do I make failFrom a "global" variable in that it can be accessed anywhere (from actionscript in frames and even movieclips)...Right now, when I try to trace failFrom in a different frame, it is "undefined".

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  • Java - how to tell class of an object?

    - by lkm
    Given a method that accepts as a parameter a certain supertype. Is there any way, within that method, to determine the actual class of the object that was passed to it? I.e. if a subtype of the allowable parameter was actually passed, is there a way to find out which type it is? If this isn't possible can someone explain why not (from a language design perspective)? Thanks Update: just to make sure I was clear void doSomething(MyType myType) { //determine if myType is MyType OR one of its subclasses } Since the method signature specifies the parameter as being MyType, then how can one tell if the object is actually a subtype of MyType (and which one).

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  • Abstract attributes in Python

    - by deamon
    What is the shortest / most elegant way to implement the following Scala code with an abstract attribute in Python? abstract class Controller { val path: String } A subclass of Controller is enforced to define "path" by the Scala compiler. A subclass would look like this: class MyController extends Controller { override val path = "/home" }

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  • How to count JavaScript array objects?

    - by Nikita Sumeiko
    When I have a JavaScript array like this: var member = { "mother": { "name" : "Mary", "age" : "48" }, "father": { "name" : "Bill", "age" : "50" }, "brother": { "name" : "Alex", "age" : "28" } } How to count objects in this array?!I mean how to get a counting result 3, because there're only 3 objects inside: mother, father, brother?!

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  • How to design app to be modular / support plugins

    - by Lee
    I'm currently in the process of refactoring my webplayer so that we'll be more easily able to run it on our other internet radio stations. Much of the setup between these players will be very similar, however, some will need to have different UI plugins / other plugins. Currently in the webplayer I do something like this in it's init(): _this.ui = new UI(); _this.ui.playlist = new Playlist(); _this.ui.channelDropdown = new ChannelDropdown(); _this.ui.timecode = ne Timecode(); etc etc This works fine but that blocks me into requiring those objects at run time. What I'd like to do is be able to add those based on the stations needs. Basically my question is, do I need to add some kind of "addPlugin()" functionality here? And if I do that, do I need to constantly check from my WebPlayer object if that plugin exists before it attempts to use it? Like... if (_hasPlugin('playlist')) this.plugins.playlist.add(track); I apologize if some of this might not be clear... really trying to get my head wrapped around all of this. I feel I'm closer but I'm still stuck. Any advice on how I should proceed with this would be greatly appreciated. Thanks in advance, Lee

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  • Calling overridden method from within overriding method in OO PHP

    - by paddymcc
    Working in a symfony model, I want to override a function and call the overridden function from within the overriding one, along the lines of class MyClass extends BaseMyClass { function setMyProperty($p) { parent::setMyProperty($p); //do some other stuff } } This is resulting in a segmentation fault. I don't want to alter the parent class - it's been generated by symfony, and may feasibly be overwritten in the future if the model is rebuilt. This seems like something that should be straightforward, but I'm struggling to find the solution. Thanks for any advice

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  • Is there any short way to load data to the properties of a class, for each column name matching the properties of the class?

    - by Ugur Gümüshan
    I want to load data to an instance of an object using its constructor and I write $this->property=$row["colname"] each time for each property. the mysql_fetch_object function fetches the data as an object but I am not sure if the instance of an object can be assigned to some object from inside. othwerwise I would use __construct($object) { $this=$object; } //doesn't give any syntax error Maybe I should look into iteration of properties and use foreach($object as $key => $value) $value=$object[$key]; or can I assign like $this=$object; within the constructor?

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  • Method return values and exceptions

    - by dnagirl
    I have an interface called iIncident which defines a single method when(). when() should return a DateTime object. I'm trying to decide what to do if $object->when() has no DateTime to return as might be the case just after an object is instantiated and before all its properties are set. My choices are: return false throw some kind of Exception return some default DateTime like '9999-01-01' My inclination is to go with an Exception since $object really can't act as an incident until it knows when it occurred. I don't want to return a default DateTime because it complicates comparisons and it's not true. And I don't really want to return false because then I have to check for it every time I call the method- but if that is the preferred method, I guess I will. Is throwing an exception the best way? And is there a predefined exception type I should use (none of the SPL ones struck me as particularly appropriate- but that might just indicate my lack of experience)?

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  • Why is my class method not visible when I implement an interface in my class?

    - by Phsika
    i can not see MyLoad.TreeLoader(.... but why i can not see? i implemented iloader to TreeViewLoad. i should see TreeLoader why? namespace Rekursive { public partial class Form1 : Form { public Form1() { InitializeComponent(); } private void Form1_Load(object sender, EventArgs e) { //treeView1.Nodes.Add("Test"); iloader MyLoad = new TreeViewLoad(); MyLoad.loader("test", treeView1, 1); // i can not see MyLoad.TreeLoader(.... but why i can not see? // i implemented iloader to TreeViewLoad. i should see TreeLoader why? //TreeViewLoad myloader = new TreeViewLoad(); } } interface iloader { void loader(string nodeName, TreeView myTre, int id); } class TreeViewLoad : iloader { public void TreeLoader(TreeView tre) { // i will call loader... } public void loader(string nodeName, TreeView myTre, int id) { myTre.Nodes.Add(nodeName + id.ToString()); if (id

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  • In what circumstances are instance variables declared as '_var' in 'use fields' private?

    - by Pedro Silva
    I'm trying to understand the behavior of the fields pragma, which I find poorly documented, regarding fields prefixed with underscores. This is what the documentation has to say about it: Field names that start with an underscore character are made private to the class and are not visible to subclasses. Inherited fields can be overridden but will generate a warning if used together with the -w switch. This is not consistent with its actual behavior, according to my test, below. Not only are _-prefixed fields visible within a subclass, they are visible within foreign classes as well (unless I don't get what 'visible' means). Also, directly accessing the restricted hash works fine. Where can I find more about the behavior of the fields pragma, short of going at the source code? { package Foo; use strict; use warnings; use fields qw/a _b __c/; sub new { my ( $class ) = @_; my Foo $self = fields::new($class); $self->a = 1; $self->b = 2; $self->c = 3; return $self; } sub a : lvalue { shift->{a} } sub b : lvalue { shift->{_b} } sub c : lvalue { shift->{__c} } } { package Bar; use base 'Foo'; use strict; use warnings; use Data::Dumper; my $o = Bar->new; print Dumper $o; ##$VAR1 = bless({'_b' => 2, '__c' => 3, 'a' => 1}, 'Foo'); $o->a = 4; $o->b = 5; $o->c = 6; print Dumper $o; ##$VAR1 = bless({'_b' => 5, '__c' => 6, 'a' => 4}, 'Foo'); $o->{a} = 7; $o->{_b} = 8; $o->{__c} = 9; print Dumper $o; ##$VAR1 = bless({'_b' => 8, '__c' => 9, 'a' => 7}, 'Foo'); }

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  • Am I trying to Implement Multiple Inheritance. How can I do this.

    - by Shantanu Gupta
    I have created a class say A which has some functions defined as protected. Now Class B inherits A and class C inherits B. Class A has private default constructor and protected parameterized constructor. I want Class B to be able to access all the protected functions defined in Class A but class C can have access on some of the functions only not all the functions and class C is inheriting class B. How can I restrict access to some of the functions of Class A from Class C ? EDIT: namespace Db { public Class A { private A(){} protected A(string con){assign this value} protected DataTable getTable(){return Table;} protected Sqlparameters setParameters(){return parameter;} } } namespace Data { public Class B:A { protected B():base("constring"){} protected DataTable output(){return getTable();} protected sqlparameter values(param IDataParameter[] parameter){} } } namespace Bsns { public Class C:B { protected C():base(){} protected DataTable show() {return values(setparameter());} } } EDIT I think what I am trying to do here is Multiple inheritance. Please check. Class A { //suppose 10 functions are declared } Class B:A { //5 functions declared which are using A's function in internal body } Class C:B { //using all functions of B but require only 4 functions of A to be accessible by C. }

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  • Extend DOMElement object

    - by Comma
    How could I exdend objects provided with Document Object Model? Seems that there is no way according to this [issue][2]. class Application_Model_XmlSchema extends DOMElement { const ELEMENT_NAME = 'schema'; /** * @var DOMElement */ private $_schema; /** * @param DOMDocument $document * @return void */ public function __construct(DOMDocument $document) { $this->setSchema($document->getElementsByTagName(self::ELEMENT_NAME)->item(0)); } /** * @param DOMElement $schema * @return void */ public function setSchema(DOMElement $schema){ $this->_schema = $schema; } /** * @return DOMElement */ public function getSchema(){ return $this->_schema; } /** * @param string $name * @param array $arguments * @return mixed */ public function __call($name, $arguments) { if (method_exists($this->_schema, $name)) { return call_user_func_array( array($this->_schema, $name), $arguments ); } } } $version = $this->getRequest()->getParam('version', null); $encoding = $this->getRequest()->getParam('encoding', null); $source = 'http://www.w3.org/2001/XMLSchema.xsd'; $document = new DOMDocument($version, $encoding); $document->load($source); $xmlSchema = new Application_Model_XmlSchema($document); $xmlSchema->getAttribute('version'); I got an error: Warning: DOMElement::getAttribute(): Couldn't fetch Application_Model_XmlSchema in C:\Nevermind.php on line newvermind

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  • In what circumstances are instance variables declared as '_var' in 'use fields' readonly?

    - by Pedro Silva
    I'm trying to understand the behavior of the fields pragma, which I find poorly documented, regarding fields prefixed with underscores. This is what the documentation has to say about it: Field names that start with an underscore character are made private to the class and are not visible to subclasses. Inherited fields can be overridden but will generate a warning if used together with the -w switch. This is not consistent with its actual behavior, according to my test, below. Not only are _-prefixed fields visible within a subclass, they are visible within foreign classes as well (unless I don't get what 'visible' means). Also, directly accessing the restricted hash works fine. Where can I find more about the behavior of the fields pragma, short of going at the source code? { package Foo; use strict; use warnings; use fields qw/a _b __c/; sub new { my ( $class ) = @_; my Foo $self = fields::new($class); $self->a = 1; $self->b = 2; $self->c = 3; return $self; } sub a : lvalue { shift->{a} } sub b : lvalue { shift->{_b} } sub c : lvalue { shift->{__c} } } { package Bar; use base 'Foo'; use strict; use warnings; use Data::Dumper; my $o = Bar->new; print Dumper $o; ##$VAR1 = bless({'_b' => 2, '__c' => 3, 'a' => 1}, 'Foo'); $o->a = 4; $o->b = 5; $o->c = 6; print Dumper $o; ##$VAR1 = bless({'_b' => 5, '__c' => 6, 'a' => 4}, 'Foo'); $o->{a} = 7; $o->{_b} = 8; $o->{__c} = 9; print Dumper $o; ##$VAR1 = bless({'_b' => 8, '__c' => 9, 'a' => 7}, 'Foo'); }

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